Picture size control circuit for television receivers



June 8, 1948. FOSTER I 2,443,030

PICTURE SIZE CONTROL CIRCUIT FOR TELEVISION RECEIVERS Filed Nov. 9, 1946 2| PICTURE SIGNAL I Inventor": Raymond F.Fost,er-,

mbm' H is Attorney.

Patented June 8, 1948 PICTURE SIZE CONTROL CIRCUITFOR TELEVISION RECEIVERS Raymond F. Foster, Stratford, Conn, assignor to General Electric Company, a corporation of New York Application November 9, 1946, Serial No. 709,034

4 Claims.

My invention relates to television receiver circuits and particularly to circuits for supplying potentials to the anode and beam deflecting coils of a cathode ray tube employed in a television receiver.

It is customary in television receiver circuits to provide to the horizontal beam deflecting coils of the cathode ray tube a current which increases at a constant rate for a length of time equal to the picture signal portion of a received television signal so that the cathode ray beam travels from one side to the other of the fluorescent screen or end wall of the tube. This uniformly varying current is repeated at regular intervals spaced apart by the time required to return the beam to its starting position. It is known to utilize the pulse of voltage incident to the current employed to return the cathode ray beam to its starting position to produce a high potential which may be rectified and supplied to the anode of the cathode ray tube. When the same transformer is employed in both the horizontal beam deflecting generating circuit and the anode voltage sup,- ply circuit, difiiculty is encountered in controlling the amplitude of the horizontal beam defleeting current to control the size of the television picture without afiecting undesirably the magnitude of the anode voltage. When an attempt is made to control picture size by increasing or decreasing the horizontal driving current, a change appears also in the voltage supply circuit to produce an undesired change in picture size which nullifies the result produced by the change in driving current magnitude.

It is the primary object of the present invention to provide a new and improved anode voltage supply circuit and horizontal beam deflecting circuit in which the magnitude of the beam deflecting current may be varied over a substantial range without affecting substantially the intensity of anode voltage.

It is still another object of my invention to provide a new and improved circuit for controlling the size of the picture of a television receiver.

It is a further object of my invention to provide a new and improved circuit for controlling the size of the picture of a television receiver which is simple in construction and operation.

Briefly stated, in accordance with one aspect of the present invention a transformer is employed for supplying both horizontal beam deflecting current to the picture tube of a televisionreceiver and to generate a high intensity anode voltage for that tube. The intensity of the beam deflecting current is controlled to regulate the size of the picture by means of a variable inductance connected in series with the secondary winding of the transformer. The variation of inductance in the secondary winding circuit is effective to introduce a change of incuctance in the primary winding circuit sufficient to compensate for any undesirable changes in the anode potential supply circuit otherwise caused by the change in the primary winding current incident to the variation of the load in the secondary winding circuit.

For a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawing and its scopewill be pointed out in the appended claims. In the drawing, the single figure is a circuit diagram of a portion of a television receiver suitably embodying my invention.

Referring to the drawing, there is shown an output power tube I which applies current to the horizontal deflecting coils 2, 3 of the cathode ray tube 4 through a transformer 5. The power tube l is of the tetrode type having a cathode 6 which is connected to ground through a cathode resistor 7; Control electrode 8 of tube 1 is connected to an input terminal 9 through a coupling capacitor l0 and to ground through a grid resistance H. The anode l2 of the tube l is connected to an intermediate point I3 on the primary winding comprising coils [4, I5 of transformer 5. The lower terminal of the coil 14 is supplied with operating potential from the usual anode supply indicated by the legend B+. The screen electrode [6 of tube 1 is connected to the lower terminal of winding I4 for unidirectional currents and is by-passed to ground through a capacitor I! for alternating currents.

Preferably, the input terminal 9 is supplied with a sawtooth wave of voltage, such as the wave l8, from preceding portions of the television receiver. In the usual television receiver circuit, the duration of each sawtooth wave corresponds to the horizontal scanning period and adjacent of the'sawtooth waves are separated by a pulse of voltage having a duration corresponding to the usual horizontal retrace period. The power tube I utilizes the input voltage wave M5 to produce, in its anode circuit, a sawtooth wave of current. The voltage wave across the winding l4 generated by this sawtooth current is as indicated at H). The wave I9 comprises slightly decreasing sawtooth portions separated by high positive pulses of a duration corresponding to the duration of the horizontal sweep retrace rectifier and a capacitor 22 connecting the cathode to ground.

Since the wave of voltage i-9, which is across the primary winding of transformer 5, contains positive pulses, the wave of current in secondary winding 23 of transformer 5 is not entirely sawtooth in form, but varies therefrom at periods following the positive peaks by shock oscillations. vide to the deflection coils 2, 3 a sawtooth wave of current 24, I provide a damping tube 25 to supply a compensating current to the horizontal deflection coils. The control electrode 26 of the damping tube 25 is connected through a re sistance 21 to a differentiating circuit comprising a capacitor 28 and a variable resistor 29. The cathode 30 of damping tube 25 is connected to ground through a load resistor 3| shunted by a by-pass capacitor 32. Bias for the control electrode of the damping tube 25 is obtained by grid voltage rectification and grid current in resistor 33 shunted by a by-pass capacitor 3 3. One terminal of resistor 33 is connected to the cathode 36 and the opposite terminal is connected to a variable tap 35 on resistor 29. In this circuit, the differentiation of the elements 28, 29 is varied by varying the position of tap 35 to control the amount of compensating current supplied to the deflection coils by tube 25.

In order that the magnitude of the current supplied to the deflection coils may be varied to control the size of the picture produced on the cathode ray tube 4, I provide a variable inductance 36 which is connected in series between the lower terminal of secondary winding 23 and ground. Preferably, the inductance 36 has a relatively low Q and may be shunted if desired by a resistance 3'! to reduce the amount of oscillatory energy stored in the inductance 36.

In the operation of the above-described circuit, the size of the picture on the fluorescent screen or end wall of the cathode ray tube 4 may be controlled by varying the inductance 36. The effect of this variation is either to increase or decrease the magnitude of the sawtooth current supplied to the horizontal deflection coils 2, 3. Such a change in current in the secondary winding produces an undesirable change in the current flow in the loading of the primary Winding of transformer 5 which tends to reduce the magnitude of the voltage supplied to the anode of the cathode ray tube. Such a reduction of anode voltage, however, is compensated by the change of inductance in the secondary circuit which is transferred to the primary circuit to vary the reactance thereof in a direction to maintain the anode supply voltage at a desired value. Such a result is in contrast to that obtained by attempts to regulate the size of the picture by using an inductance or resistor in shunt with a portion of the secondary Winding 23, the effect of which is opposite to that produced by the inductance 38 and which tends to produce achange of high voltage which nullifies the change in picture size produced by such a shunt circuit.

An important advantage of my improved cir- To eliminate such variations and to pro-' cuit for controlling the size of the picture of a cathode ray tube of a television receiver circuit is that the inductance required for size control is relatively small and, furthermore, has no deleterious effect upon the anode voltage supply circuit.

A still further advantage lies in the fact that the variable size control inductance 36 is connected into the horizontal sweep coil circuit in advance of the variable tap 35 so that the linearity of the sweep field may be satisfactorily maintained by controlling the position of the tap 35 on resistance 29.

While I have illustrated my invention as including various devices diagrammatically shown,

it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention and I, therefore, aim to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of my invention,

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The combination, in a television receiver circuit, including a cathode ray tube having an anode and a beam deflecting coil, a transformer having a primary winding and a secondary winding, means supplying a sawtooth wave of current to a portion of said primary winding, means utilizing the voltage across said primary Winding to provide a unidirectional potential to said anode, said coil being connected in series circuit with said secondary winding, and variable reactance means included in said series circuit for varying the current in said coil without changing the intensity of said potential to control the size of a picture produced on said cathode ray tube.

2. The combination, in a television receiver circuit, including a cathode ray tube having an anode and a beam deflecting coil, a transformer having a primary winding and a secondary winding, means supplying a sawtooth wave of current to a portion of said primary winding, means utilizing the voltage across said primary Winding to provide a unidirectional potential to said anode, said coil being connected in series circuit with said secondary winding, and means for varying the current in said coil without changingthe intensity of said potential comprising a variable impedance connected in said series circuit.

3. In a television receiver circuit, the combination comprising a cathode ray tube having an anode and a beam deflecting coil, a transformer having a' primary winding and a secondary winding, means supplying a substantially sawtooth wave of current to said primary Winding, means utilizing the voltage across said primary winding to provide a. unidirectional potential to said anode, a variable inductance and said coil being connected in series circuit with said secondary winding, the size of the picture produced on said cathode ray tube being variable by variation of said inductance, and said inductance being of sufficient value to maintain said potential at a substantially constant value as the current in said transformer is varied to control the size of a picture,

4. The combination, in a television receiver circuit, of a cathode ray tube having an anode and a horizontal beam deflecting coil, a transformer having a primary Winding and a sec- 5 6 ing, means for rectifying the voltage across said primary winding to provide a unidirectional po- REFERENCES CITED tential to said anode, said coil being connected h win references are of record in t in series circuit with said secondary winding, file Of this P nt: means connected in shunt with said secondary 5 UNITED STATES PATENTS winding for compensating for non-linearities 1n the current thereof to provide a sawtooth wave Number Name Date of current to said coil, and reactance means 10741495 Vance 1937 connected in said series circuit for controlling 2,183,647 3115,55 1940 the magnitude of the current supplied to said 10 2,212,217 Whlte et a1 20,

coil.

RAYMOND F. FOSTER. 

